1. Field of the Invention
The present invention relates generally to CMOS image sensors and in particular to a CMOS image sensor provided with an on-chip Read Only Memory (ROM).
2. Brief Description of the Related Art
Image sensing devices are known that employ light detecting elements such as photodiodes or photo-transistors formed using complementary metal oxide semiconductor (CMOS) techniques. The imaging devices generally include an array of pixels containing the light detecting elements arranged on a chip.
Typically, an integrated circuit imaging device formed using a CMOS process has an image sensor pixel array with associated row and column conductors and decoders, an analog signal path including a sample and hold circuit for reading pixel signals from the column conductors, a digital to analog converter for digitizing the analog pixel signals, a controller for controlling array read out operations, a pixel processor for processing digital pixel signals, and an input/output (I/O) port all fabricated on a single chip. The controller manages operation of the read out of the image sensor array, the analog signal path and the analog digital converter, and may also operate as the pixel processor.
During manufacture, each imaging pixel array is usually tested individually. Tests detect defective pixel circuits, pixel signal level, and other array attributes, and the information is stored based on lot and individual device identification numbers. The information developed during testing can be utilized to enhance the operation of the device by, for example, compensating for defective pixels, differing pixel signal levels, and other tested pixel attributes.
U.S. Pat. No. 6,396,539 to Heller et al. discloses a CMOS imager with an on-chip programmable memory for storing pixel and identification information for a CMOS sensor array. The preferred embodiment disclosed by Heller et al., is based on a flash memory, which is complex and bulky. It would be desirable to provide an image sensor having on chip programmable read only memory employing a simple and compact circuit for reading and writing data, and which requires no difficult processing for forming the memory. A simple and easy way to store pixel array information on-chip for later use is also desirable.